1. Field of the Invention
This invention relates generally to a DC-DC converter, and more particularly is directed to a DC-DC converter suitable for use with a radio receiver which is operable by a battery.
2. Description of the Prior Art
In a radio receiver of the electronic tuning type, that is, a receiver having a tuning circuit which includes a varicap diode and in which channel selection is carried out by varying a voltage supplied to the varicap diode, the channel selection voltage must be varied over a suitable range, for example, from 0.5 to 4.5 V. When the receiver is operated by a 3 V or 6 V battery, and in consideration of the fact that the battery is progressively consumed, the channel selection voltage is provided by a DC-DC converter.
Various types of DC-DC converters are known, for example, a ringing choke type converter (RCC) as shown in FIG. 1, and an oscillation transformer type as shown in FIG. 2. In the ringing choke type converter shown in FIG. 1, a transistor Qa is switched on or off by a PWM (pulse width modulation) pulse from a PWM modulator circuit 11 to thereby deliver an output voltage to a terminal Ta. This output voltage is supplied to a voltage comparator circuit 12 in which it is compared with a variable reference voltage Er. The duty ratio of the PWM pulse from modulator circuit 11 is controlled by the compared output from voltage comparator circuit 12 so as to make constant the output voltage at terminal Ta. The constant output voltage at terminal Ta is changed by changing the reference voltage Er.
In the oscillation transformer type converter shown in FIG. 2, an oscillation is carried out by a resonance frequency between a transformer Ma and a capacitor Ca to develop an output voltage at a terminal Ta and the peak level or magnitude of an oscillation from an oscillation circuit 13 is controlled by a compared output from a comparator circuit 12, whereby the output voltage at the terminal Ta is made constant. In this case, the output voltage at terminal Ta may be changed by adjusting a variable resistor Ra having a movable tap to which terminal Ta is connected.
Since the converter shown in FIG. 1 is of a switching type, it generates a large noise which will badly disturb the receiver. Accordingly, it is not suitable to provide the channel selection voltage for the varicap diode by means of this switching type converter.
If the converter shown in FIG. 2 is not provided with the variable resistor Ra, the output voltage is controlled by varying the oscillation level or magnitude of circuit 13. In this case, it is difficult to stably control the oscillation level or magnitude over a wide range for achieving a corresponding wide range of channel selection voltages. Particularly when the output voltage is made low, the oscillation is stopped. To this end, and as shown in FIG. 2, the oscillation level or magnitude is made constant, and the desired output voltage is obtained by using the variable resistor Ra. However, with this latter circuit arrangement, even when the output voltage is to be at a minimum, the oscillation circuit 13 must keep the oscillation at an oscillation level or magnitude corresponding to that required for the maximum output voltage with the result that the conversion efficiency of the DC-DC converter is lowered. Of course, this is not desirable for a receiver which is operated by battery.